Coral reefs

Orange Tube Coral (Tubastrea faulkneri), Wild Guide to Moreton BayCoral reefs are iconic ecosystems of Queensland, extending along nearly the entire coastline and out into the Coral Sea. The Great Barrier Reef is the largest coral reef in the world and the largest structure ever built by living organisms – visible even from outer space. They are places of high density living, like human cities, built by trillions of soft, tiny coral individuals (polyps). These polyps absorb calcium carbonate from the seawater to build stony skeletons, from which they get their name as so-called hard corals (hexacorals). While it is the hard skeleton that forms the fabric of coral reefs, the living tissue is usually confined only to the thin outer veneer, and the bulk of the subsurface is mostly dead limestone from previous generations of polyps constantly building on top of each other.

Toenail Egg Cowrie (Calpurnus verrucosus), Wild Guide to Moreton Bay

Coral polyps feed by capturing microscopic creatures from the seawater using their tentacles and special stinging cells (cnidoblasts), but many corals that build reefs (scleractinians) also have microscopic plants (zooxanthellae) living in their tissues which use sunlight to photosynthesise and provide substantial extra energy. These plants also give corals their characteristic bright colours. This symbiosis give corals a major competitive advantage over most other marine sessile invertebrate species (animals attached to the seabed for at least part of their life cycle), providing the ability for coral reefs to grow more rapidly, and corals to compete for space better than most marine invertebrates. The symbiosis is a fragile one, however, and when stressed (overheating, changes to water chemistry etc), corals will expel these zooxanthellae and consequently turn white (coral bleaching).

Other groups of animals included in the corals, such as sea anemones, zoantharians and octocorals (soft corals, sea pens and gorgonians) do not build solid limestone skeletons but have colonies enclosed by soft living tissue. Corals reproduce both asexually – by clonal fragmenting, with fragments growing into new colonies – and sexually – through mass annual spawning, in late spring or early summer, when trillions of eggs and sperm are broadcast into the water, are fertilized externally, and float in the plankton until they find new places to settle and grow.